Liquid crystal displays (LCD's) are rapidly becoming the universally dominant display technology. Over 250 million large, i.e. >10-inch, LCD flat-panel displays (FPDs) are built annually for the manufacture of televisions, desktop computer monitors and notebook computers, and over 1 billion smaller, i.e. <10-inch, displays are built annually for other applications, such as mobile phones and personal digital assistants (PDAs). Since LCD is a passive, i.e. non-emissive, technology, the display assembly usually requires a backlighting unit (BLU) in order to function in the application. Accordingly, there is a very strong demand for BLUs, e.g. annual BLU sales are approximately $14 billion and growing strongly as FPDs rapidly displace conventional cathode ray tube (CRT) technology in virtually all applications.
At the current state-of-the-art, the BLU is typically constructed using cold-cathode fluorescent (CCFL) tubes mounted in a complex arrangement that results in a very costly subassembly. In fact, the BLU for a typical LCD display today accounts for 25% to 35% of the bill-of-materials (BOM) cost. Therefore, there is a very strong motivation in the industry to find alternative methods of backlighting that reduce the BOM cost. In addition, the CCFL component contains mercury, which is classed as a toxic substance, creating a further motivation for displacing CCFL.
In order to produce a color image, the state-of-the-art LCD technology must incorporate a color filter (CF) component, which must be finely aligned with the LCD pixels. The color filter is another very costly element, accounting for some 20% of the BOM cost. In addition, the color filter degrades the contrast ratio and, more importantly, degrades the efficiency of the display by more than a factor of three, thus adding to the backlight cost and seriously impacting the energy efficiency of the FPD and therefore of the end-product.
Despite the serious cost and performance shortcomings of conventional BLU technologies that have been in use and incrementally improved over the last twenty-five years, the industry has so far been unable to create a viable alternative, which adequately addresses the fundamental issues of cost and efficiency.
FIGS. 1, 2 and 3 illustrate the construction of a conventional LCD display, including polarizers 1 mounted on the front and back of the display for filtering incoming and outgoing light, i.e. only passing light of a certain polarization into the liquid crystal and only passing light of a different polarization out of the LCD. Sheets of glass 2 are provided for sandwiching the liquid crystals therebetween and providing substrates for the remaining elements. Seals 3 and spacers 4 provide the necessary distance between the sheets of glass 2 and contain the liquid crystal therebetween. Transparent electrodes 5, e.g. a transparent conductive oxide (TCO) such as indium tin oxide (ITO), enable an electric field to be applied to the liquid crystal for altering the polarization of light passing therethrough, and therefore the amount of light that is able to pass through the top polarizer 1. A hard coat layer 6 and a polyimide film 7 provide protective coatings for the TCOs 5. Thin-film transistor (TFT) control elements 8 define a matrix of independently addressable pixels through which the passage of light is controlled. A color filter (CF) layer 9 is comprised of a matrix of alternating red, green and blue filters enabling the color of the transmitted light to be controlled. The color filter layer 9 is the major cost driver in this assembly, accounting for some 20% of the bill-of-materials (BOM) cost of the FPD and also has very low optical efficiency, reducing the brightness of the display by 75%.
FIG. 2 illustrates the construction of a typical BLU that is mounted against the LCD assembly in order to build the complete FPD panel. Multiple elements are required in order to distribute the light from the CCFL tube 10 for the required brightness and uniformity including a cylindrical reflector 11 and a flat reflector 12. A wedge-shaped light guide panel (LGP) 13 redirects the light at a 90° angle through a diffuser sheet 14, a vertical prism sheet 15, a horizontal prism sheet 16, and a protective sheet 17. The BLU is typically supplied to the FPD maker as a complete subassembly that accounts for some 25%-35% of the BOM cost of the FPD.
An alternative method of backlighting, disclosed in U.S. Pat. No. 5,121,234 issued Jun. 9, 1992 to Kucera, consists of placing a panel of electro-luminescent (EL) material immediately behind the LCD display. This method can have a relatively low cost and enable a relatively simple and thin assembly compared with the CCFL approach. However, conventional EL technology, despite decades of development, has not been able to achieve brightness levels much beyond 100 cd/m2, which is two orders of magnitude less bright than required in applications such as televisions and computer displays. Furthermore, even the best available EL materials have been unable to achieve anywhere close to the color gamut required in those applications or even in small color display applications, such as mobile phones. Therefore, EL technology at the current state-of-the-art is suitable as backlighting only for displays where high brightness and wide color gamut are not requirements, such as monochromatic displays for instrumentation or backlights for mobile phone keypads.
An object of the present invention is to overcome the shortcomings of the prior art by providing a backlighting arrangement that reduces the existing BOM cost and also provides a substantial efficiency improvement.